FIG. 5 illustrates a conventional forklift 100. The forklift 100 includes forks 3 for holding a load 2, masts 4 to which the forks 3 are attached so as to be able to ascend and descend, hydraulic cylinders 5 for performing an operation of raising/lowering the forks 3 and the masts 4, a lift lever 6 for starting/stopping the raising/lowering operation, a hydraulic device 7 for supplying hydraulic oil to the hydraulic cylinders 5 and discharging the hydraulic oil from the hydraulic cylinders 5, a control valve 8 for controlling amounts of hydraulic oil supplied and discharged, and a control device 20 for controlling the hydraulic device 7 and the control valve 8.
The control device 20 includes a current calculation portion 20A and a current supply portion 20B, as shown in FIG. 6. The current calculation portion 20A calculates a current command value on the basis of a start/stop signal outputted by the lift lever 6 and outputs a current command regarding the current command value to the current supply portion 20B. The current supply portion 20B supplies the control valve 8 with an energizing current in accordance with the current command value. Moreover, the current supply portion 20B outputs a drive signal to a motor 7C for use in driving a pump 7B, and supplies the hydraulic cylinders 5 with hydraulic oil in a tank 7A.
Incidentally, the forklift 100 has a problem where the load 2 on the forks 3 is vertically vibrated when the forks 3 starts a raising/lowering operation (particularly, a lowering operation). As a solution for this problem, there is a method in which a different vibration is generated in the load 2 after the raising/lowering operation is started, thereby offsetting the vibration caused at the start of the raising/lowering operation (see, for example, Patent Document 1).
Described below is an example where the solution is applied when an operation of lowering the forks 3 is started. The lift lever 6 is shifted by an operator over a period from time t10 to time t11, as shown in FIG. 7(A), and when a tilt angle of the lift lever 6 reaches X (e.g., a maximum tilt angle) at time t11, the operation of lowering the forks 3 is started.
Once the forks 3 start descending at time t11, a first vibration is generated at the center of gravity G of the load 2, as shown in FIG. 7(B). In this case, by generating a second vibration at the center of gravity G of the load 2 at time t12, the first vibration can be reduced by offsetting. Preferably, the second vibration is 180° out of phase with the first vibration and has the same amplitude as the first vibration (strictly, the second vibration has a smaller amplitude by an amount of attenuation, as shown in FIG. 7(B)).
In the case of the forklift 100, to generate the second vibration at time t12, the current calculation portion 20A increases the current command value in two steps, as shown in FIG. 7(C). Specifically, the current command value is gradually increased from 0 to B11 (one half of B12) over a period from time t11 to time t11′ and is maintained at B11 from time t11′ until time t12 before being gradually increased from B11 to B12 over a period from time t12 to time t12′. As a result, the energizing current supplied to the control valve 8 is gradually increased in two steps in accordance with the current command value, so that the forks 3 gradually descend in two steps.